[ARC5] 6k8 - 12k8?

[Dennis Monticelli]

Here's how to find out (assuming you don't want the tube).  Cut off the
envelope at the base with a hacksaw or dremel, cut the base connections to
the filament, pull the filament wires out of the cathode sleeve, unfold,
inspect and then do an ohm meter test.  If it's a 12V filament, the unfolded
wire should yield the correct ohmic value while the visual inspection could
yield clues.

Dennis AE6C

On Sat, Apr 16, 2011 at 10:22 AM, J. Forster <jfor at quik.com> wrote:

> That makes a whole lot more sense. I think tubes were always tested before
> shipment and a 6 V tube would not survive 12 V for very long at all.
>
> -John
>
> =================
>
>
> > I have another theory.  That may be a correctly built 12K8 with half the
> > filament shorted upon itself.  Your typical filament is folded (often
> more
> > than once) before it is inserted into the cathode sleeve.  When the
> > insulating oxide coating on the filament breaks down or flakes off, it
> > sometimes makes contact with the sleeve (heater to cathode leakage) but
> it
> > can also make contact with its folded self.
> >
> > Dennis AE6C
> >
> > On Fri, Apr 15, 2011 at 1:39 PM, Leslie Smith <vk2bcu at operamail.com>
> > wrote:
> >
> >> Dear list,
> >> I have a valve (or "toob") marked 12k8.  Manufacturer: KEN-RAD,  The
> >> cold filament resistance is around 3.9 to 4 ohms.  This is out of the
> >> set, unconnected.  Is this a 6k8 that is mis-labelled?  My experience to
> >> date has been that the cold resistance of any of the VT13x "toobs" is
> >> around 12 ohms.
> >>
> >> I'm uncertain about the source of this valve.  It came from a box with
> >> valves, IF transformers, chassis screws etc that was in another box
> >> containing a "scrap" BC-453 I am in the process of rescuing from a
> >> morbid fate.  In other words I don't know where it came from.
> >>
> >> I found it in the following manner.  Last night I was at the stage of
> >> re-fitting the valves to the "scrap" chassis; I inserted a pair of
> >> valves, turned the filament supply "on" and measured the voltage at the
> >> junction of the two filaments.  In this case, that turned out to be
> >> about 20 volts.  Hmmmm.  (Thinks ... this is unusual, Mr. Ohm is doing
> >> strange things ....)  Do various things - make various checks.  Mutter
> >> about socket resistance and promise to never wash another chassis in
> >> Dot's Bath Cleaner.  Got another 12k8 (this one was made by RCA) and
> >> measured the cold filament resistance.  Around 12 ohms.  Put this 12K8
> >> (by RCA) into the chassis, and measured the voltage at the center point
> >> of the two filaments.  Near enough to 12 volt.  (Thinks:  Dot's bath
> >> cleaner may not be so bad after all.)
> >>
> >> I imagine this valve somehow got from the 6K8 batch into the 12K8 batch
> >> before the labelling machine at KEN-RAD.  Any-one seen this before?  Or
> >> maybe it's just a broken filament in a 12K8?
> >>
> >> 73 greetings from Australia
> >>
> >> Les
> >>
> >> --
> >>  Leslie Smith
> >>  vk2bcu at operamail.com
> >>  (end)
> >>
> >> --
> >> http://www.fastmail.fm - Send your email first class
> >>
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